This invention relates generally to an improved gas sensor, and in particular to an improved gas sensor which detects the presence of gas by the change in threshold voltage of a MOS transistor.
Recently, highly accurate, inexpensive gas sensors have become popular and several have been commercialized. These gas sensors have been used for the purpose of detecting the presence of gas in order to prevent a gas explosion. They also have been used in homes or public establishments to indicate existence of a fire. In these situations the gas sensors is relied upon to determine the presence of gas. A defective sensor can cause an accident. Thus, a highly accurate sensor is required. Further, in veiw of the popularization of the gas sensors for home use, a relatively inexpensive gas sensor is desired.
Having these objectives in mind, the following types of conventional gas sensors have been available. First, a detector was based on combustion of a gas utilizing platinum as a catalyst. This type of detector deteriorated with time as the catalyst deteriorated and the gas alarm could not be generated unless the gas concentration is maintained. In addition, the detecting voltage is extremely low, so that a complicated bridge circuit is required in this type of detector. This results in an expensive detector and further it is not possible to select a gas to be detected.
Another type of detector utilized to overcome these short comings utilizes a sintered metal-oxide-semiconductor. This type of detector is highly sensitive to gases and a simplified circuit structure can be used therein. However, since the sensor element generally must be heated up to a temperature of several hundred degrees centigrade in order to improve the sensitivity, the electric power consumption is considerably high. In addition, the stability of the sensor element is also subject to some deterioration with time. As with the platinum catalyst detectors, the problem of a lack of ability to select a gas to be detected has not been overcome.
Alternatively, a detector may be based on using a photoelectric method or a smoke sensor may be based on an ionization method. However, both methods are poor in their ability to distinquish the gas to be detected, are large and bulky in size. In the latter or ionization type of smoke sensor, a radioisotope is utilized as an ionization source raising the question that the radiation may be harmful to humans.
Recently, a gas sensor utilizing a MOS transistor in a new method has been proposed as an alternative to the above-noted methods. This is to say, by employing a material which absorbs gases and is sensitive thereto as a gate electrode of the transistor, the change in threshold voltage due to the existence of the gas is detected. In this method, the sensor element can be massproduced inexpensively by known IC manufacturing techniques. In addition, a sensor element of high quality and uniform properties can be readily manufactured. Moreover, the sensor element can be miniturized, so that a driving circuit can be formed on the same substrate and the power consumption can be reduced.
The only shortcoming in this new method is that the sensor element utilized is not sensitive to gases because a heating member which can be utilized in other methods cannot be employed in the MOS method in view of the deleterious long term effects on the reliability of the MOS transistor. Accordingly, it is desirable to provide a MOS transistor gas sensor having sufficient sensitivity to gases at room temperature so as to be practical. The gas is detected by examining the relationship between the absorption of gases and the threshold voltage change in a MOS. A highly sensitive sensor can be provided when the threshold voltage change due to absorption and desorption of the gases is extremely large.